Skip debugger tests on non-trio backends

tests/test_debugger.py

@@ -42,10 +42,16 @@ def mk_cmd(ex_name: str) -> str:
 
 @pytest.fixture
 def spawn(
+    start_method,
     testdir,
     arb_addr,
 ) -> 'pexpect.spawn':
 
+    if start_method != 'trio':
+        pytest.skip(
+            "Debugger tests are only supported on the trio backend"
+        )
+
     def _spawn(cmd):
         return testdir.spawn(
             cmd=mk_cmd(cmd),